1. Field of the Invention
This invention relates generally to the field of playback of digital video as in a digital video tape recorder (VTR) or virtual digital VTR. More particularly, this invention relates to performing variable speed playback and shuttle of compressed digital video material in a manner which emulates the performance of a video tape recorder's variable speed playback and shuttle. The present invention is also useful in instances where temporal compression is needed (i.e. judicious merging elements of video frames to compensate for smaller display interval of a rapid video frame display).
2. Background of the Invention
In video tape recorders, variable speed playback of the video material is generally accomplished by increasing the tape speed during playback and providing dynamic tracking. In video tape editing application, such variable speed playback has a distinctive look which editors are accustomed to.
When a moving video image is stored digitally and retrieved at a higher than normal playback rate, this is generally accomplished by simply deleting (or not retrieving) frames of information. For example, with reference to FIG. 1, if the ten sequential frames F.sub.1 through F.sub.10 which are shown in FIG. 1 are to be played back at twice the normal playback rate, only alternate frames might be displayed. For example, frames F.sub.1, F.sub.3, F.sub.5, F.sub.7 and F.sub.9 would be displayed at the normal frame rate while frames F.sub.2, F.sub.4, F.sub.6, F.sub.8 and F.sub.10 would be omitted entirely. Consequently, only five frames of information would be displayed at a standard frame rate requiring only half the display time. Thus, the illusion that the image has been displayed at twice normal speed is created.
Unfortunately, when non-contiguous frames of digital video are consecutively displayed to produce a high speed image, a rough jumpy display results resembling a montage or a teleconferencing-type image in which updates are slower than real time resulting in discontinuous video. This type of implementation thus diverges substantially from the look of a video tape recorder's variable speed playback. In a video tape recorder, the motion is smooth and continuous but occurs at a higher rate. At high speed shuttle rates wherein discontinuous frames of video are consecutively displayed, the human brain has difficulty tracking the movement and little information is thus conveyed to the user.
It is desirable to create a simulation of this variable speed playback such as appears in analog VTR equipment for use in technology such as optical, magneto optical, magnetic and other storage media wherein, in general, the video frames are randomly accessible and not necessarily temporally ordered (e.g., frame 1 of a video sequence could be adjacent to frame 20 in a video sequence). Applications such as disk based nonlinear editors for digital video are appropriate candidates for such technology.